A problem in the manufacture of films, such as photographic films, is that accurate real time control over the manufactured thickness thereof is difficult to achieve. In fact, it is estimated that thirty (30%) percent or more of manufactured photographic film is typically out of specification, and must be reprocessed because film manufacturing systems have no reliable, accurate means by which to continuously monitor and control, in real time, manufacturing process results. It should be appreciated that were such a reliable, accurate means by which to continuously monitor film thickness during manufacture thereof available, a feedback control system could be fashioned to effect variation in manufacture process parameters, as required, to keep film thickness within specifications during manufacture and/or handling thereof.
In light of the problem identified infra herein, it is disclosed that the present invention provides a very reliable and accurate Method to continuously monitor the "Total Film Retardance" of a Birefringent Film, (eg. for the purposes of this Disclosure, "Total Film Retardance" is defined as the product of the difference in the Indicies of Refraction ((N1)-(N2)) in the two directions of Refringence in a Film, multiplied with Film Thickness (T), (.alpha.N.multidot.T)). Note that "Total Film Retardance" also implies that a total number of degrees of Retardance is measured, rather than simply some cyclic value between zero (0.0) and three-hundred-sixty (360) degrees. That is, the sum total number of degrees of retardance in numerous sequential cycles can be measured. It should also be appreciated that the present invention does not monitor film thickness per se., but provides indication thereof if changes in Birefringent Refractive Index Optical Properties of a Film are not dominating, with respect to the effects of thickness change in a monitored product of: (the difference between Refractive Indicies in a Birefringent Film multiplied by the thickness of said Birefringent Film). It is to be understood that the presence invention is particularly well suited to reliably and accurately identifying differences in "Total Film Retardance" monitored at different locations on a Birefringent Film, during, for instance, the manufacture thereof.
A search was conducted to identify relevant Patents, and a small number of Patents were identified.
A Patent to Colombotto et al., U.S. Pat. No. 4,584,476 describes a device for non-destructively testing internal stress of heat tempered glass. A system including crossed-polarizers is disclosed, between which crossed polarizers a heat tempered glass sample is positioned in use. The 476 Patent specifies the use of monochromatic infrared radiation.
A Patent to Gorman et al., U.S. Pat. No. 4,523,848 describes a Polarscope which includes first and second quarter wave plates positioned on either side of a birefringent sample under test. In use the quarter wave plates can be switched between two optical conditions to alter the polariscope between a plane polarized and a circularly polarized mode. The 848 Patent describes the use of white light.
A Patent to Simila, U.S. Pat. No. 3,807,868 describes a Method for determining fiber orientation in paper utilizing normally incident light which is reflected therefrom. Polarizers positioned at ninety degrees with respect to one another are present. The polarizers present in the 868 Patent are utilized to isolate quadrature polarized beam components in a polarized beam which reflects from a sample under test. It is also noted that use of monochromatic light is recited in the Claims.
A Patent to Krieger et al., U.S. Pat. No. 3,274,882, similar to the 476 Patent to Colombotto et al., provides apparatus for use in detecting stress in transparent glass sheets. The presence of crossed-polarizers in a light transmission pathway is described as a means to mediate light transmitted through a sheet of glass possessing laminar stress under test and monitored by a detector, as is the presence of a detector to monitor reflected light. Light transmitted through a the crossed-polarizers, and a present or absent sheet of glass therebetween, is utilized in conjunction with reflected light detection means in use.
A Patent to Gawrisch et al, U.S. Pat. No. 4,909,630 describes a system which generates a two dimensional interference image of a biaxially stretched film strip. Areas in said two dimensional interference image which demonstrate different orientation and/or thickness present as "streaks". The use of polychromatic light is described as a means to generating a colored interference image.
Finally, a German Patent, No. DT 23 38 305 A1, describes the presence of crossed-polarizers in a system utilized to detect changes in birefringence of a film placed therebetween.
No known reference, however, teaches that polychromatic light should be transmitted through a system comprised of two polarizers with a birefringent film therebetween, and that a varying intensity pattern, as demonstrated in an Intensity vs. Wavelength plot, should be subjected to mathematical analysis, such as Fourier analysis, to mediate reliable, accurate determination of changes in the "Total Film Retardance" effected by said birefringent film, in response to stresses applied thereto, such as in film manufacturing and/or handling procedures. That is, the present invention allows monitoring Peak Intensity vs. Frequency locations, and changes therein during use, in a mathematically transformed format. Said mathematical transformation serves to improve reliability and accuracy over "crossed-polarizer" system effected monitoring techniques which do not utilize such mathematical transformations, because mathematically transformed results provide more localized and pronounced Peak Intensity vs. Frequency results, to monitor in use.
In view of the identified Patents there remains a need for an improved system and method for use in monitoring total film retardance, particularly in real time during a birefringent film manufacturing process.